(1) Field of the Invention
The present invention relates to marine vehicles and more particularly to lateral thrusters for use therein.
(2) Brief Description of the Prior Art
Marine vehicles often are required to maneuver at very low speeds and hover in currents. Marine vehicles typically use rudders or other control surfaces to produce maneuvering forces. However, flow over the control surfaces is required to produce a maneuvering force and these forces vary with the square of the vehicle speed. Therefore, at low speed, control surfaces become ineffective. Typically, lateral tunnel thrusters are located in the bow or stern of marine vehicles to meet the low speed maneuvering requirements. However, the effectiveness of tunnel thruster decreases with forward velocity of the vehicle. Often there is an intermediate vehicle speed at which neither the control surfaces nor the thruster produce effective maneuvering forces.
Conventionally, thrusters make use of a rotating propeller in a tunnel through the vehicle. The rotating propeller creates a pressure differential across the blades and drives a jet of water through the tunnel and out one side. The integrated pressure force on the blades is transferred to the vehicle via the rotor hub and force acting in the opposite direction of the jet flow. This effect is used to maneuver the vehicle. In the current art thrusters are designed to be reversible and so that the vehicle may be maneuvered in either port or starboard directions.
Early efforts to measure the effects of forward vehicle velocity on tunnel thruster performance have shown that as the forward velocity was increased to speed on the order of 3 knots, the effective side force (force perpendicular to the vehicle axes) from the tunnel thruster decreased to as low as 10 percent of the side force measured at zero maneuvering effectiveness as forward vehicle velocity. Thus with the current art tunnel thruster quickly lose their maneuvering effectiveness as forward vehicle velocity increases. Experiments conducted to understand this phenomenon indicated that the forward velocity does not significantly alter the force acting on the vehicle through the propeller hubs. However, the thruster jet acts as an obstruction to the boundary layer flow over the vehicle hull. This produces a wake deficit in the boundary layer downstream of the thruster's jet. The resulting wake-induced pressure deficit on the vehicle surface generated an integrated suction force on the hull that counteracts the force on the blades. Conversely, on the suction side of the tunnel due the vehicle boundary layer being sucked off by the thruster. The integrated force in this high-pressure region also counteracts the force on the thruster blades.
Tunnel thrusters are typically reversible. That is, the blades can be rotated clockwise to produce a jet in either direction to maneuver the vehicle. Thus any device that is deployed to mitigate the effects of forward velocity must also be reversible.
Various specific arrangement of tunnel thrusters are shown in the prior art.
U.S. Pat. No. 3,408,974 to Pehrsson, for example, discloses a ship steering system which includes tunnels extending transversely through a ship's hull at the bow or stern or both in which is mounted a reversing or reversible pitch propeller in order to pump water selectively through the tunnel to exert a steering force on the hull and including vanes or screens which can be extended outwardly form and withdrawn into the hull located behind the ends of the tunnel or tunnels in the direction of movement of the ship in order to exert a turning force on the hull and also to direct water selectively into the tunnel during the forward or rearward movement of the ship to enable control of the steering of the ship either at low or high speed.
U.S. Pat. No. 3,710,748 to Baer et al. discloses a longitudinal flow passage which opens at the bow of a ship and has impeller means therein with first and second discharge flow passages branching from the longitudinal passage behind the impeller and opening on both sides of the hull. Controllable valve means in the discharge flow passages control the flow of water being discharged from openings whose rear edges project outwardly of the hull surface a distance about one fourth of the width of the discharge opening.
U.S. Pat. No. 3,830,184 to Krautkremer discloses an attachable or a detachable unit providing a lateral thrust rudder for ships. The invention contemplates a unitary mechanism constituting a tunnel, a propeller within such tunnel and driving means for same which can be bodily mounted into or detached from a ship. When same is in operating position, it is normally mounted at the bow of the ship and functions to apply a lateral thrust in one direction or the other as desired to such bow. The unit is mounted so that the driving mechanism projects into the interior of the ship for easy access thereto. Suitable drive mechanism and control features, including pitch-changing means for the propeller blades are also provided.
U.S. Pat. No. 4,008,676 to Brix discloses a water craft which has a hull with a cavity communicating with a sea opening below water level. A conduit formation in the hull has one end opening on a side of the hull directly adjacent the sea opening and below water level and is connected through the interior of the hull and has an opposite end which opens into the cavity at a spaced location from the sea opening.
U.S. Pat. No. 4,018,181 to Brix discloses a lateral thrust control unit for watercrafts having a pair of tunnels, which are directed transversely to the longitudinal axis thereof. Each of the tunnels extend from one side of the watercraft to the oppositely positioned side of the watercraft and have at least one drivable propeller therein. At least one pressure-compensating channel is provided near the tunnels and connects at least one of the zones of differing pressure fields created on the sidewalls of the watercraft as the watercraft moves simultaneously longitudinally and laterally to the pressure field of different potential to equalize the pressure differential therebetween and to reduce the resistance to the lateral movement. The pressure-compensating channels do not have any propulsion devices therein.
U.S. Pat. No. 4,214,544 to Dashew, et al. discloses an improved boat thruster including a diverter valve having an inlet connected to a water pump and a pair of outlets extending to either side of the boat. Each outlet includes a primary nozzle and a deflector movable to a first position wherein it allows water flow from the primary nozzle to be discharged to one side to thus thrust the boat to the opposite side. Each deflector is also movable to second and third positions for directing the primary nozzle water flow to respective secondary nozzles for discharging the water either forwardly or rearwardly to thus thrust the boat in the opposite direction. The secondary nozzles each have an exit area smaller than that of the primary nozzle.
U.S. Pat. No. 4,455,960 to Aker discloses an improved boat thruster system including a pump for drawing water through an inlet in the boat hull and for discharging water through first and second pipes connected to outlets located on either side of the hull. A valve is installed in each of the pipes to control the flow of water therethrough. The valves may be controlled be either an open or closed loop control system configured so as to prevent both outlet pipes from being closed at the same time during system operation. Each valve is preferably comprised of multiple vanes each of which is mounted for rotation about an off center axis such that in the event of a valve control system failure, the water flow will cause the valve to open rather than close thereby preventing undesirable high pressure buildup in the system.
U.S. Pat. No. 5,501,072 to Plancich, et al. discloses a thrust propulsion mechanism for a boat including an outlet conduit extending athwartships from a first outlet port to a second outlet port in the hull. A paddle-wheel impeller is mounted within the hull for rotation about an axis of rotation by a reversible motor. A circumferential paddle portion of the paddle-wheel impeller extends into an aperture defined centrally in the top wall of the outlet conduit. An inlet conduit extends athwartships from a first inlet port to a second inlet port, and intermediate thereof supplies water to the center of the paddle-wheel impeller. Water is discharged from the paddle-wheel impeller through one of the outlet ports, dependent on the direction of rotation of the paddle-wheel impeller, to create thrust by a combined paddle-wheel and centrifugal pump action.
U.S. Pat. No. 5,642,684 to Aker discloses an improved thrust director unit provided for discharging a directionally adjustable water jet flow from the hull of a marine vessel to generate a thrust reaction force for close-quarter maneuvering and/or propulsion of the vessel. The unit comprises a thruster housing having an outlet through which the jet flow is discharged, wherein the outlet is defined by diverging fore and aft walls to permit angularly forward or rearward jet flow discharge for vessel propulsion. At least two deflector vanes are moveable together within the housing outlet and cooperate therewith to define a directionally adjustable discharge flow path for selective jet flow discharge in a sideward direction to produce a sideward thrust, or in a forwardly or rearwardly angled direction to respectively produce a reverse or forward propulsion thrust. In the sideward thrust position, the discharge flow path has a nondiverging cross section and is isolated from the diverging fore-aft walls of the housing outlet.